Silicon sheet material or ribbon is particularly important in making low cost solar cells. Continuous growth of silicon ribbon obviates the need for slicing of bulk produced silicon to form wafers. In U.S. Pat. Nos. 4,594,229; 4,627,887; 4,661,200; 4,689,109; 6,090,199; 6,200,383; and 6,217,649, continuous silicon ribbon growth is carried out by introducing two strings of high temperature material up through a crucible that includes a shallow layer of molten silicon. The strings serve to stabilize the edges of the growing ribbon. The molten silicon freezes into a solid ribbon just above the molten layer. U.S. Pat. Nos. 6,090,199 and 6,217,649 describe a method and apparatus for continuous replenishment of the feedstock material in a continuous silicon ribbon. As presently practiced, a single ribbon is grown out of a single crucible, with each ribbon machine having one such crucible. FIG. 1 illustrates this process.
In order to produce lower cost solar cells and hence expand large-scale electrical applications of solar electricity, it is important to have lower cost substrate materials for making the solar cell. The current invention provides new and improved methods and apparatus for growing silicon ribbons.